1. Field of the Invention
The present invention relates to a non-isolated voltage sensor provided specifically between a high-voltage battery and a circuit having a pull-up resistance one end of which is connected to a cable while the other end thereof is connected to a reference power supply, and more particularly to a non-isolated type voltage sensor capable of sending a detected voltage accurately without considering a cable length.
2. Description of the Related Art
In a recent high-voltage vehicle containing a plurality of batteries of 12V or the like connected to each other, for example, there is such a trend that a plurality of wire harnesses are included in a junction box (J/B) and sensors, measuring circuits and the like are accommodated in this junction box if possible. The location of this junction box in a vehicle compartment varies depending on the vehicle type.
However, because a conventional voltage measuring circuit for use in automobile has a limitation on the wire harness length, there is such a problem that the locations of the voltage measuring circuit and battery controller in the vehicle compartment cannot be changed. Further, if the vehicle type is changed so that the location of the junction box is changed, there is another problem that the sensor and voltage measuring circuit cannot be accommodated in the junction box because the wire harness length is limited.
To solve this problem, Japanese Patent Application Laid-Open No. H9-257837 has disclosed a conventional voltage sensor which eliminates the necessity of considering the harness length.
A voltage sensor 10 shown in FIG. 1 comprises a power supply line 13 for supplying an electric power from a battery 2 to a load 12 and an isolated type detector 16 having a hall device 15 in a gap of a magnetic core 14. The power supply wire 13 passes through a through hole of the detector 16 as a primary side wire.
This voltage sensor further comprises a differential amplifying circuit 17 for differential-amplifying an output from both ends of the hall device 15, a current buffer 18 connected to this differential amplifying circuit 17 and the like. An output wire 19 of the current buffer 18 is wound around the core 14 of the isolated type detector 16 by a predetermined number of turns as a secondary side wire. Then, a voltage signal is sent to a battery controller 22 through a harness 20.
An end of an input circuit 23 of this battery controller 22 is connected to an input terminal of the voltage follower 24 and the harness 20 while the other end thereof is connected to the ground. This ground is a weak current ground which is different from a ground for the battery 2 and the voltage sensor 10.
That is, when the detector 16 is used, magnetic flux proportional to a current flowing from the battery 2 to the load 12 is generated and then, the hall device 15 detects that magnetic flux as a hall voltage, so that the differential amplifying circuit 17 and a current buffer 18 supply a current in a direction which makes zero a hall voltage to the secondary wire side.
The current flowing to the secondary wire side is sent to the input circuit 23 of the battery controller 22 through the harness 20. That is, because an output from the voltage sensor 10 becomes an output of current, the harness length can be neglected (the harness may be long).
However, because in this voltage sensor 10, the battery voltage is not detected directly, there is a problem in terms of a detection accuracy although the harness length can be neglected. Further, there is another problem that this voltage sensor 10 necessitates the core, hall device, differential amplifying circuit and the like, the quantity of the components increases.
The present invention has been achieved to solve the above described problems and therefore, an object of the invention is to provide a non-isolated type voltage sensor for a battery of vehicle in which the restriction on the harness length is mild, the non-isolated type voltage sensor being capable of measuring a voltage accurately without use of the hall device.
The non-isolated type voltage sensor of the present invention is a non-isolated type voltage sensor provided between a battery and a circuit containing a pull-up resistor which is connected to a reference power supply and a cable.
This non-isolated type voltage sensor comprises a voltage dividing circuit, a voltage follower and a voltage-current conversion circuit.
The voltage dividing circuit is comprised of plural resistors connected in series and connected to the battery in parallel. Then, a voltage proportional to a terminal voltage of the battery and less than the voltage of the reference power supply is obtained at a desired voltage dividing point of these plural resistors.
Because the voltage follower converts a high input impedance to a low input impedance, it takes a role as a buffer circuit for removing respective influences between the voltage dividing circuit and the voltage-current circuit. Therefore, if a voltage at the voltage dividing point of the voltage dividing circuit is inputted to the voltage follower, a low impedance output voltage is obtained.
Next, the voltage-current conversion circuit supplies a current proportional to a low impedance output voltage obtained through the voltage follower to the ground through the cable and output resistor.
This voltage-current conversion circuit comprises: a field effect transistor a source of which is connected to the cable while a drain thereof is connected to the output resistor; an output resistor which is connected to a drain of the field effect transistor and the ground, the output resistor having a resistance higher than a resistance of the pull-up resistor; and an amplifier in which an output of the voltage follower is connected to a non-inverted input thereof, an inverted input thereof is connected to the output resistor and an output thereof is connected to a gate of the field effect transistor.
Thus, if the voltage dividing circuit obtains a voltage proportional to the battery voltage at the voltage dividing point, the field effect transistor is turned ON immediately through the voltage follower.
That is, because a current supplied to the gate of the field effect transistor is made very small when the field effect transistor is turned ON, even if the voltage at the voltage dividing point is converted to current, the current flowing to the ground through the cable becomes a current accurately proportional to a detection current at the voltage dividing point.
Further, the non-isolated type voltage sensor of the present invention is a non-isolated type voltage sensor provided between a battery and a circuit having a first pull-up resistor which is connected to the ground and the cable.
This non-isolated type comprises a voltage detecting circuit and a voltage-current conversion circuit.
The voltage detecting circuit obtains a voltage proportional to a battery voltage at the voltage dividing point and supplies a first current proportional to a low impedance output voltage obtained from a voltage at this voltage dividing point to the ground through a first output resistor.
The voltage-current conversion circuit supplies a second current proportional to a voltage signal determined based on the first current to the cable using a second output resistor.
This non-isolated type voltage sensor is provided between a circuit which is connected to the same type of ground as ground of the battery and the cable and the first pull-up resistor and which has a differential amplifying circuit carrying out differential amplification with the ground as a reference and the battery.
The voltage detecting circuit comprises a voltage dividing circuit, a voltage follower and a first voltage-current conversion circuit. The first voltage-current conversion circuit comprises: a first field effect transistor a source of which is connected to an input of the second voltage-current conversion circuit while a drain thereof is connected to the output resistor; a first output resistor which is connected to a drain of the first field effect transistor and the ground; and a first amplifier in which an output of the voltage follower is connected to a non-inverted input thereof, an inverted input thereof is connected to the output resistor and an output thereof is connected to a gate of the field effect transistor.
The voltage detecting circuit further comprises a second voltage-current conversion circuit. The second voltage-current conversion circuit comprising: a second pull-up resistor which is connected to the reference power supply and a source of the first field effect transistor; a second output resistor which is connected to the reference power supply; a second field effect transistor a source of which is connected to the second output resistor while a drain thereof is connected to the cable; and a second amplifier in which a non-inverted input thereof is connected to a source of the first field effect transistor, a non-inverted input thereof is connected to the second output resistor and a source of the second field effect transistor and an output thereof is connected to a gate of the second field effect transistor.
As described above, in the non-isolated type voltage sensor of the present invention, a high voltage of the battery is detected by the voltage dividing circuit so as to obtain a voltage proportional to a low impedance output voltage obtained by converting impedance. Consequently, if this non-isolated type voltage sensor is provided between a high voltage battery and a circuit having a pull-up resistor which is connected to a cable and the reference power supply, a current proportional to the detected voltage flows from this cable to the output resistor and ground in succession immediately.
That is, because the detected voltage signal is supplied as a current signal to the cable, a current signal proportional to the voltage signal is obtained at a subsequent circuit even if the cable is long. As a result, there is an effect that freedom of the layout of the non-isolated type voltage sensor in the vehicle compartment is increased.
Further, because the voltage-current conversion circuit employs the field effect transistor, a current for driving the gate can be minimized. Thus, there is an effect that an accurate current proportional to the detected voltage can be obtained even if the cable is long.
Further, in the non-isolated type voltage sensor provided between a high voltage battery and a circuit having a first pull-up resistor which is connected to a cable and the ground, a first voltage-current conversion circuit having a first field effect transistor and a second voltage-current conversion circuit having a second field effect transistor are provided. Then, a voltage signal corresponding to a current flowing through the first field effect transistor by the second pull-up resistor connected to the reference power supply in the second voltage-current conversion circuit is generated and this voltage is converted to current by the second voltage-current conversion circuit and sent to a subsequent circuit.
As a result, a stabilized current signal is sent to the subsequent stage, so that a difference of the current signal can be amplified with respect to a common ground to the battery. Consequently, there is an effect that a highly accurate differential signal can be obtained.
Further, there is an effect also that the circuit can be produced in a small size because no hall device is necessitated.